In general, in a disk brake, the braking force is produced by piston-cylinder units formed in a suitable caliper and each comprising a cylinder in which a piston can slide so as to be able to act, by means of pads, on opposed surfaces of a rotating disk.
The piston-cylinder unit typically has a flexible boot for protecting the cylinder and the piston (particularly their sliding surfaces) against the entry of foreign material and/or corrosive agents such as, for example, dust, dirt, or moisture, which may prevent the piston sliding correctly inside the cylinder during a braking operation and/or may give rise to corrosion of the sliding surfaces of the piston and of the cylinder.
The flexible boots normally in use are bellows-shaped and such a boot is fitted on the piston-cylinder unit by fixing one of its ends around the piston and an opposite end around the cylinder so as to form a protective barrier which extends around the piston-cylinder unit in the region of the opening of the cylinder.
When the flexible protective boot is in the rest condition or when it is fitted in a disk brake in which the piston is disposed entirely inside the cylinder, as occurs with pads which are not worn, the boot adopts a configuration in which some of its portions are rolled up around one another so as to form one or more bends or pleats. It should be noted that, when the flexible boot is fitted on a disk brake, these pleats have convex portions facing towards the pad and, in particular, towards a support plate of a friction lining of the pad. As a result of wear of the friction linings of the pads, the piston extends towards the disk and the flexible boot stretches out to adapt to the new position adopted by the piston.
It is known that, during the assembly of the disk brake and, in particular, during the installation of the piston-cylinder unit and its flexible protective boot, a certain amount of air may remain trapped in the space between the boot and the piston-cylinder unit.
During a braking operation, because of the overheating of the disk on which the pad acts, heat is emitted and is transmitted to the trapped air by the piston, increasing the pressure which the air exerts on the flexible boot. This pressure may cause expansion of the flexible boot such as to bring its pleats or other portions of the boot into direct contact with the pad and, in particular, with the support plate. This contact is particularly likely in disk brakes having new, that is, unworn friction linings, in which the piston is substantially entirely retracted into the cylinder and the convex portions of the pleats of the flexible boot are close to the support plate.
It is pointed out that, as a result of the braking operation and, in particular, if this operation is prolonged, the pad is at a high temperature and may therefore burn or in any case damage the flexible boot with which it is in contact, necessitating its replacement if the problem is noticed.
Moreover, the problem of the contact between the flexible boot and the pad is particularly marked in disk brakes using floating calipers which require flexible boots of considerable extent so that the boots have several pleats that come close to the pad.
A method proposed by the prior art for preventing this damage provides for the use of a flexible boot having a suitable hole for the outlet of trapped air. According to this technique, the discharge of the air prevents bulging of the boot and hence contact with the pad.
As is known from U.S. Pat. No. 4,809,821, a further method provides for the formation, in the vicinity of the cylinder, of a suitable annular channel in which the pleats forming the flexible boot can be housed completely so that they remain a suitable distance from the pad.
However, it has been found that the methods proposed up to now do not prevent contact between the flexible protective boot and the pad or the consequent burning which damages the boot.
In fact, because of the high temperatures which may be reached during braking, the use of air-outlet holes does not suffice entirely to prevent the expansion of the air and of the boot trapping it.
It has also been noted that contact between the boot and the pad is not only brought about by the above-mentioned expansion of the air but may be due to incorrect positioning of the pleats which form the boot. For example, an incorrect arrangement of the pleats of the boot may occur during assembly by the manufacturer or, more probably, as a result of the refolding of the flexible boot following the replacement of the pads by a maintenance person. In fact, the replacement of the pads, which requires the piston to be returned into the cylinder, is an operation which may be performed in conditions such as not always to ensure correct refolding of the boot because of the limited space which is left free by the pads that have been removed.
It should also be borne in mind that, as well as complicating and weakening the structure of the caliper in which the cylinder is formed, the presence of a suitable channel for completely housing the flexible boot does not solve the problem created by incorrect mounting of the boot or by the bulging brought about by the heating of trapped air.